Rechargeable lithium-ion (“Li-ion”) batteries often use expensive and toxic materials, such as cobalt. In addition, current lithium-ion batteries using high-capacity and highly energy dense electrode materials typically fail after a relatively small number of charging and discharging cycles. Some currently existing Li-ion technologies have energies of about 230-270 Wh/kg at a price point of about $300/kWh. However, the Department of Energy and the automotive industry have targeted an improvement in energy to 400 Wh/kg at a price point of approximately $200/kWh. Incorporating high-energy materials may help to improve energy and lower price. Such Li-ion configurations may include a Silicon (Si) anode and a nickel-rich nickel-manganese-cobalt (NMC) or lithium-manganese-rich (LMR) cathode.
Silicon is one of the most attractive high-energy anode materials for use in lithium-ion batteries. Silicon has a low working voltage and high theoretical specific capacity of 3579 mAh/g, nearly ten times higher than that of currently known graphite electrodes. Despite these advantages, a silicon anode has serious disadvantages that discourage its use in a commercial battery. One of these disadvantages is associated with silicon's severe volume expansion during lithiation. While a commercialized graphite electrode expands roughly 10-13% during lithium intercalation, silicon's expansion is nearly 300%, generating structural degradation and instability of the solid-electrolyte interphase (SEI). Instability in the solid-electrolyte interphase in a silicon anode shortens the battery life to levels that render it unattractive for commercialization.
Degradation of the silicon active material has been mitigated by incorporating nanoscale materials including nanoparticles, nanowires, core-shell nanowires, nanotubes, yolk-shell nanoparticles, pomegranate structures, nanoporous structures, and/or nanocomposites. However, the size (below 500 nm in diameter) of these materials, their processing requirements, and the elaborate nano-architectures required for their use in anodes mean that they cannot be produced by commercially viable processes. It is desirable to develop a truly scalable silicon anode capable of effectively utilizing larger, low-cost active material particles while still achieving excellent battery cycling performance. Unfortunately, previous attempts to use large silicon particles showed fast capacity decay or utilized complex tailored binders to mitigate the electrode degradation.
Moreover, the aforementioned nano-material electrode architectures, despite providing significant improvements to silicon electrode performance, lack the needed coulombic efficiency largely because the volume change during silicon alloying and de-alloying renders the solid-electrolyte interphase at the silicon-electrolyte interface mechanically unstable. The solid-electrolyte interphase layer forms on the anode surface through reductive decomposition of the electrolyte during charging of the battery. Silicon anodes suffer extensively from a dynamic solid-electrolyte interphase that must reform each cycle as expansion during lithiation causes the layer to break. Formation of the solid-electrolyte interphase consumes lithium ions and depletes electrolyte during every cycle. Alternative electrolyte compositions and active material surface treatments have been studied in the effort to enhance solid-electrolyte interphase formation on high-capacity anode materials and improve half-cell coulombic efficiency. In spite of these efforts, the coulombic efficiency achieved throughout cycling is still insufficient for a long lasting silicon-based full-cell.
Lithium-manganese-rich (LMR) layered oxides, also known as over-lithiated oxides (OLO), are of interest as cathode materials for lithium-ion batteries given their high capacities (greater than 250 mAh/g) and energy densities. A commonly studied over-lithiated oxide material is formulated as (x)Li2MnO3(1-x)LiR1O2(R1=Mn, Ni, Co) and is often described as being composed of layered Li[Li1/3Mn2/3]O2 (generally designated as Li2MnO3) and LiR1O2 with a specific capacity of ˜250 mAh/g. It has been proposed that these materials are composed of two phases, namely a parent trigonal layered LiR1O2 phase (space group [R-3m]) with monoclinic Li2MnO3-like (space group [C2/m]) components. This material may be referred to as both a “layered-layered” composited and a “solid solution.”
Despite their high specific capacities, these materials are susceptible to rapid capacity fade due to the evolution of the Li2MnO3 and LiR1O2 parent structures towards a spinel phase during electrochemical cycling. This effect also results in a lower operating voltage, thereby damaging the energy density of the cell (often referred to as “voltage fade”). During the first charge cycle, this phase change is known to occur at the surface of the electrode particles in combination with oxygen evolution as Li2O is lost from the Li2MnO3 parent structure. During subsequent cycles, the layered to spinel phase change continues from particle shell to core, accompanied by the dissolution of Mn (Mn2+). While the phase change occurring during the first cycle is seen as an “activation” step, the long-term phase change of the lithium-manganese-rich layered oxide material causes a gradual lowering in operating voltage of the cell and capacity degradation, rendering the material inadequate for utilization in lithium-ion batteries.
Various strategies have been employed in order to counteract the phase change in this material including doping the crystals with alkali atoms in an attempt to support the lithium layers or reducing the manganese content to restrict formation of the Mntetragonal phase. However, these methods have found little success.
Ni-rich NMC materials also suffer from capacity fade due to metal leaching. These materials are also unstable at high temperatures and highly exothermic, which may lead to explosions in the presence of conventional electrolytes. The nickel-rich chemistries exhibit structural degradation and thermal instabilities; these problems worsen with higher nickel content, higher temperature, and higher cutoff voltages (>4.4 V vs. Li/Li+). In general, Ni-rich cathode materials with a layered structure undergo structural degradation from the layered R-3m phase to the spinel-like Fd-3m phase and the rock-salt Fm-3m phase. This structural change is caused by the migration of the transition metal ions into the lithium layer during charge/discharge cycling. The metal ion migration leads to the layered-to-spinel phase transformation, and this transformation is exacerbated by cycling conditions such as high voltage and high temperature due to the increasing number of vacant Li sites during full delithiation and the diffusion of transition metal ions. This structural change leads to capacity fade (decreasing number of Li vacancies for intercalation and active material loss due to metal dissolution). Moreover, the decomposition of organic electrolytes at high voltages leads to higher interfacial resistances and increased rates of structural degradation.
Conventional efforts aimed at solving the aforementioned problems with the LiNixM1-xO2 material may attempt to passivate the electrode-electrolyte interface using complex surface modifications, often with a conductive polymer. Despite resulting in improvements in cycling stability, such techniques do not address thermal instabilities/safety and are not sufficient for commercial application, which requires high stability for over 300 cycles.